Development and Structure of the Root Cortex in Caltha palustris L. and Nymphaea odorata Ait

Structural features of the mature root cortex and its apoplastic permeability to dyes have been determined for two dicotyledonous wetland plants of differing habitats: Nymphaea odorata, growing rooted in water and mud, and Caltha palustris, growing in temporal wetlands among cattails. In mature roots, movement of the apoplastic dyes, berberine and safranin, into the roots was blocked at the hypodermis, indicating the presence of an exodermis. A hypodermis with an exodermis, i.e. Casparian bands in the outermost uniseriate layer plus suberin lamellae, is present in both species. In N. odorata, hypodermal walls are further modified with cellulosic secondary walls. Roots of N. odorata and C. palustris have an endodermis with Casparian bands only. A honeycomb aerenchyma is produced by differential expansion in N. odorata and includes astrosclereids and diaphragms, while roots of C. palustris have no aerenchyma, but some irregular lacunae are found in old roots. These aspects of cortex structure are related to an open meristem organization, with unusual patterns of cell divisions in certain ground meristem cells (called semi-regular hexagon cells) of N. odorata. The correlation between aerenchyma pattern and hypodermal structure appears to be related to habitat differences.